System level architectures for relayed downlink (DL) communications have been widely studied where communication is configured in time division multiplexing (TDM) fashion. Data is transmitted to a wireless transmit/receive unit (WTRU) or a relay station (RS) in different time slots.
In a system including a WTRU and a RS that has a flat fading channel condition, various relaying schemes have been proposed for DL cellular communications, and simulations of high speed downlink packet access (HSDPA) have been performed. It has been shown that these proposed relaying schemes greatly boost a cell-edge WTRU's signal-to-interference and noise ratio (SINR) distribution, and extend the cell coverage. These schemes assume a two-hop communication, which consists of two phases. In phase 1 (T1), a base station (BS) transmits a message, intended for a WTRU, to a selected RS, until the RS correctly decodes the message. After the RS succeeds in decoding a received message sent by a BS, the RS takes over the communication and forwards information needed for the WTRU to fully decode the message. This transmission may be performed with or without the cooperation of the BS.
Two main channel coding techniques have been proposed for these relaying schemes. A first main channel coding technique is conventional fixed-rate coding. A second main channel coding technique is rateless coding.
The basic concept of rateless coding is to encode an input message with an infinitely long block length. The (presumably infinite) output is then divided into an infinite stream of finite-sized messages which are then transmitted sequentially. A transmitter terminates the transmission of the rateless codeword only when it receives an acknowledgement (ACK) of successful decoding from a receiver. Note that no actual transmission is ever infinite, since a truly infinite code output cannot be generated in practice. Rather, either a new coded message is generated from the input “on the fly” (as needed), and/or a finite number of retransmissions is established as a system parameter. However, the actual rate of the rateless code is determined by the time when the receiver decodes the message correctly, and is not fixed. This enables a rateless code to adapt to whatever conditions a time-variable channel, (e.g., a fading channel), may present and effectively pick the correct coding rate for that channel. Alternatively, it is known that it is difficult for a fixed-rate coded system to do so without channel state information (CSI) being available in the transmitter. Furthermore, a fixed-rate coded system is forced to operate at low efficiency, (i.e., low rate), to combat channel variation and increase channel reliability. These factors alone suggest the application of rateless coding in a relaying system. However, a rateless coding paradigm is also a natural paradigm for distributing coding between multiple terminals, (WTRU and RS), and it is desired the terminals to transmit cooperatively with a minimal amount of synchronization.
In uplink (UL) communication, the network configuration and interference modeling may be the same as in DL communication. There may be one transmitter antenna and one receiver antenna each, in the WTRU, the RS, and the BS.
A number of RSs may be dedicated to assist one WTRU in each cell. The RS may be associated with a BS. The RSs may only assist one WTRU, which may be associated with the same cell and in communication with the associated BS.
There may be one WTRU per cell, and the number of RSs assisting the WTRU at one time slot may be limited to one.
Similar to DL, let b denote the total number of information bits that the WTRU has to deliver to the BS. In phase 1, the BS may not receive all b bits sent from the WTRU or may not receive any information bits at all. Let the subscript r denote transmissions to the relay, from the relay, or both. Let u denote transmissions from the WTRU. The rate from the WTRU to the RS in phase 1 is denoted Rr1. The rate from the RS to the BS is denoted Rr2. The rate from the WTRU to the BS in phase t is denoted as Rut, where tε{1, 2}. Let P′rt represent the average received power at the receiver of the transmission involving the RS in Phase t, and let P′ut represent the average received power at the BS sent from the WTRU in phase t. Let gxt and Ixt represent the fading coefficient and inter-cell interference (ICI) which occurred in transmission x, in phase t. Let ρxt denote the ICI power spectrum which occurred in transmission x, in phase t and let ρ0 denote the noise power spectrum. Let N0 denote the white noise power occupying the whole band.
In a unicast two-hop scheme, wherein the WTRU transmits to the RS, may be used. During phase 1, a WTRU sends information to a selected RS using up to the maximum rate supportable by the WTRU-RS link. Generally, the number of information bits sent to the RS is b. A split cooperative multiplexing scheme enables the WTRU to send bRS (a subset of b bits).
During phase 2 (T2), for transmissions from the RS to the BS using forwarding, the RS takes over the transmission and forwards the information bits to the BS.
For transmissions from the RS and the WTRU to the BS using coherent cooperative diversity, CSI may be available at the transmitters. The transmitters may use the channel phase information feedback from the receiver to multiply their signals with a phase-weight, and enable their signals to add coherently at the receiver. The total number of bits cooperatively transmitted by the transmitters is b bits.
For transmissions from the RS and the WTRU to the BS using Alamouti cooperative diversity, an Alamouti transmission scheme may be enabled among the RS and the WTRU. The RS and the WTRU cooperatively transmit the b bits to the BS to achieve the best diversity and multiplexing trade-off.
For transmissions from the RS and the WTRU to the BS using a simple cooperative Multiplexing scheme, the RS and the WTRU act as two distributed antennas. The b bits of information are encoded by the WTRU and the RS independently to guarantee the per-link capacity, and transmitted to the BS.
For transmissions from the RS and the WTRU to the BS using a split cooperative multiplexing scheme, wherein b=bRS+bWTRU, the RS and the WTRU act as two distributed antennas. The WTRU pushes new information bits, bWTRU, to the BS, and the RS transmits bRS bits to the BS. The RS and the WTRU use two different codewords to encode bRS and bWTRU information bits respectively. The per-link capacity is guaranteed by this transmission.
In a multicast two-hop scheme, wherein the WTRU transmits to the RS, the BS, or both, may be used. During phase 1, the WTRU sends information to the selected RS and BS with the maximum rates, (R_{r1} and R_{m1}, respectively), supportable by the respective WTRU-RS and WTRU-BS links. Generally, the number of information bits sent to the RS is b. A split cooperative multiplexing scheme enables the WTRU to send bRS, (a subset of b bits). The WTRU-BS link may be worse than the WTRU-RS link. In this case, the RS may send an acknowledgement (ACK) before the BS does. During this period, the BS only decodes fractional information sent to the RS, (i.e., b1 bits, which is the subset of b or bRS).
During phase 2, for transmissions from the RS to the BS using forwarding, the RS forwards the remaining b2 (where b2=b−b1) information bits, which have not been received by the BS.
For transmissions from the RS and the WTRU to the BS using coherent cooperative diversity, CSI may be available at the transmitters. The transmitters may use the channel phase information to multiply their signals with a phase-weight, and enable their signals to add coherently at the receiver. The total number of bits sent from the transmitters is b2 (where b2=b−b1) bits.
For transmissions from the RS and the WTRU to the BS using Alamouti cooperative diversity, the RS and the WTRU cooperatively transmit the remaining b2 bits to the BS with an Alamouti transmission scheme. By doing so, two levels of diversity can be obtained without losing any data rate.
For transmissions from the RS and the WTRU to the BS using a simple cooperative multiplexing scheme, the RS and the WTRU multiplex the remaining b2 bits with two different codewords and send those bits to BS.
For transmissions from the RS and the WTRU to the BS using a split cooperative multiplexing scheme, wherein b=bRS+bWTRU, the new information bits, bWTRU, are pushed from the WTRU to the BS directly. The RS and the WTRU act as two distributed antennas and use two different codewords to encode bRS and bWTRU bits respectively. The per-link capacity is guaranteed by this transmission.
A quasi-static flat fading channel may be considered. Due to the protocol symmetry of one WTRU-one RS-one BS communications in DL and UL, the relaying schemes for DL one WTRU-one RS communication can be applied in UL one WTRU-one RS communication, as discussed above.
The respective rates for each link in different transmission phases can be similarly derived.
Cell-edge WTRUs obtain more benefits from relaying communications than cell-center WTRUs. Therefore, similar to DL communications, cell-edge WTRUs usually seek RS help to increase channel link reliability. Unlike DL communication, the first hop communication in UL relaying systems experiences more ICI than the second hop. This is due to the geometry of the cell-edge WTRUs. Therefore, the ICI impact on the overall performance of UL relaying systems may be different from the DL relaying systems.
Consideration may be given to WTRU and BS locations, and how to pick up the available RS such that the throughput for a given scheme is maximized.
Consideration is given to the best relay location, which provides the largest throughput for UL communications. Multicast split cooperative multiplexing schemes may provide the best performance among the DL relaying schemes, without requiring CSI at the transmitter. The corresponding scheme for UL is illustrated in FIG. 1.
The effective rate for the multicast split cooperative multiplexing scheme is expressed as:
                                                                                          R                                                            Multicast                      ⁢                      _                      ⁢                      Split                                        ⁢                                          _                      ⁢                      Coop                                        ⁢                                          _                      ⁢                      Mux                                                                      =                                ⁢                                                      b                    +                                          b                      3                                                                                                  T                      1                                        +                                          T                      2                                                                                                                                                                =                                    ⁢                                                                                                              R                                                      r                            ⁢                                                                                                                  ⁢                            1                                                                          ⁡                                                  (                                                                                    R                                                              r                                ⁢                                                                                                                                  ⁢                                2                                                                                      +                                                          R                                                              u                                ⁢                                                                                                                                  ⁢                                2                                                                                                              )                                                                    -                                                                        R                                                      u                            ⁢                                                                                                                  ⁢                            2                                                                          ⁢                                                  R                                                      u                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                              R                                                  r                          ⁢                                                                                                          ⁢                          2                                                                    +                                              R                                                  r                          ⁢                                                                                                          ⁢                          1                                                                    -                                              R                                                  u                          ⁢                                                                                                          ⁢                          1                                                                                                                    ,                                                    ⁢                                  ⁢        where                            Equation        ⁢                                  ⁢                  (          1          )                                                              R                          r              ⁢                                                          ⁢              1                                =                                    W              1                        ⁢                          log              (                              1                +                                                                            g                                              r                        ⁢                                                                                                  ⁢                        1                                            2                                        ⁢                                          P                                              r                        ⁢                                                                                                  ⁢                        1                                            ′                                                                                                  N                      0                                        +                                          I                                              r                        ⁢                                                                                                  ⁢                        1                                                                                                        )                                      ,                            Equation        ⁢                                  ⁢                  (          2          )                                                              R                          u              ⁢                                                          ⁢              1                                =                                    W              1                        ⁢                          log              (                              1                +                                                                            g                                              u                        ⁢                                                                                                  ⁢                        1                                            2                                        ⁢                                          P                                              u                        ⁢                                                                                                  ⁢                        1                                            ′                                                                                                  N                      0                                        +                                          I                                              u                        ⁢                                                                                                  ⁢                        1                                                                                                        )                                      ,                            Equation        ⁢                                  ⁢                  (          3          )                                                              R                          r              ⁢                                                          ⁢              2                                =                                    W              1                        ⁢                          log              (                              1                +                                                                            g                                              r                        ⁢                                                                                                  ⁢                        2                                            2                                        ⁢                                          P                                              r                        ⁢                                                                                                  ⁢                        2                                            ′                                                                                                  N                      0                                        +                                          I                                              r                        ⁢                                                                                                  ⁢                        2                                                                                                        )                                      ,                                  ⁢        and                            Equation        ⁢                                  ⁢                  (          4          )                                                  R                      u            ⁢                                                  ⁢            2                          =                              W            1                    ⁢                                    log              (                              1                +                                                                            g                                              u                        ⁢                                                                                                  ⁢                        2                                            2                                        ⁢                                          P                      2                      ′                                                                                                  N                      0                                        +                                                                  g                                                  r                          ⁢                                                                                                          ⁢                          2                                                2                                            ⁢                                              P                                                  r                          ⁢                                                                                                          ⁢                          2                                                ′                                                              +                                          I                                              u                        ⁢                                                                                                  ⁢                        2                                                                                                        )                        .                                              Equation        ⁢                                  ⁢                  (          5          )                    
If service implementation capabilities (SIC) are implemented in the receiver of the BS, the interference from the weaker link (WTRU-BS) may be cancelled in the reception of the stronger link (RS-BS).
The upper bound of Equation (1) may be achieved when Rr1=Rr2 as:
                              R                                    Multicast              ⁢              _              ⁢              Split                        ⁢                          _              ⁢              Coop                        ⁢                          _              ⁢              Mux                                      =                                                                                                  R                                          r                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      R                                          r                      ⁢                                                                                          ⁢                      2                                                                      +                                                      R                                          u                      ⁢                                                                                          ⁢                      2                                                        ⁢                                      R                                          r                      ⁢                                                                                          ⁢                      1                                                                      -                                                      R                                          u                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      R                                          u                      ⁢                                                                                          ⁢                      2                                                                                                                    R                                      r                    ⁢                                                                                  ⁢                    1                                                  +                                  R                                      r                    ⁢                                                                                  ⁢                    2                                                  -                                  R                                      u                    ⁢                                                                                  ⁢                    1                                                                        ≤                                                                                R                                          r                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      R                                          r                      ⁢                                                                                          ⁢                      2                                                                      +                                                      R                                          u                      ⁢                                                                                          ⁢                      2                                                        ⁢                                      R                                          r                      ⁢                                                                                          ⁢                      1                                                                      -                                                      R                                          u                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      R                                          u                      ⁢                                                                                          ⁢                      2                                                                                                                    2                  ⁢                                                                                    R                                                  r                          ⁢                                                                                                          ⁢                          1                                                                    ⁢                                              R                                                  r                          ⁢                                                                                                          ⁢                          2                                                                                                                    -                                  R                                      u                    ⁢                                                                                  ⁢                    1                                                                                ⁢                      =                                          R                                  r                  ⁢                                                                          ⁢                  1                                            =                              R                                  r                  ⁢                                                                          ⁢                  2                                                              ⁢                                                                                          R                                          r                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      R                                          r                      ⁢                                                                                          ⁢                      2                                                                      +                                                      R                                          u                      ⁢                                                                                          ⁢                      2                                                        ⁢                                      R                                          r                      ⁢                                                                                          ⁢                      1                                                                      -                                                      R                                          u                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      R                                          u                      ⁢                                                                                          ⁢                      2                                                                                                                    2                  ⁢                                      R                                          r                      ⁢                                                                                          ⁢                      1                                                                      -                                  R                                      u                    ⁢                                                                                  ⁢                    1                                                                        ⁢                          <                                                R                                      u                    ⁢                                                                                  ⁢                    1                                                  <                                  R                                      r                    ⁢                                                                                  ⁢                    1                                                                        ⁢                                          R                                  r                  ⁢                                                                          ⁢                  2                                            +                              R                                  u                  ⁢                                                                          ⁢                  2                                            -                                                                                          R                                              u                        ⁢                                                                                                  ⁢                        1                                                              ⁢                                          R                                              u                        ⁢                                                                                                  ⁢                        2                                                                                                  R                                          r                      ⁢                                                                                          ⁢                      1                                                                      .                                                                        Equation        ⁢                                  ⁢                  (          6          )                    
Therefore, to achieve the maximum rate of Equation (1), given a distance between the WTRU and the BS, the RS should be selected as Rr1=Rr2. Cell-edge WTRUs obtain more benefits from communicating through the RS than cell-center WTRUs. Considering cell-edge WTRUs, two-hop transmissions experience more ICI in the first hop than the second hop. For a fixed WTRU-BS distance, the RS closer to the WTRU provides higher throughput than the RS closer to the BS.
Re-organizing the upper-bound of Equation (6):
                              R                                    Multicast              ⁢              _              ⁢              Spli              ⁢              t                        ⁢                          _              ⁢              Coop                        ⁢                          _              ⁢              Mux                                      <                                                            R                                  r                  ⁢                                                                          ⁢                  2                                            +                              R                                  u                  ⁢                                                                          ⁢                  2                                                                    ︸                              S                ⁢                                                                  ⁢                1                                              -                                                                      b                                      R                                          r                      ⁢                                                                                          ⁢                      1                                                                      ⁢                                  R                                      u                    ⁢                                                                                  ⁢                    1                                                                                                b                                      R                                          u                      ⁢                                                                                          ⁢                      2                                                                                        ︸                                      S                    ⁢                                                                                  ⁢                    2                                                                        .                                              Equation        ⁢                                  ⁢                  (          7          )                    
Multicast split cooperative multiplexing schemes approach the rate which is expressed as the difference between S1 and S2. S1 represents the rate of a 2×1 multiple-input multiple-output (MIMO) scheme, meaning that the RS and the WTRU simultaneously transmit two independent data streams to the BS with different codewords. S2 represents the corresponding rate, which is calculated by dividing the number of bits received by the BS in phase 1, by in the time required for the transmission, where the bits received at the RS were re-transmitted from the WTRU in phase 2. Therefore, the rate upper bound of the split cooperative multiplexing scheme is equivalent to the rate achieved by the distributed 2×1 MIMO scheme with the transmission of b bits, subtracting the rate of the transmission of a subset of b bits received by the BS in phase 1 and redundantly transmitted by the WTRU in phase 2.
One way to increase the overall transmission bandwidth, without suffering from increased signal corruption due to radio-channel frequency selectivity, may be the use of multicarrier transmissions. In multicarrier transmissions, instead of transmitting a wider-band signal, multiple, more narrow-band signals, which are referred to as subcarriers, are frequency multiplexed and jointly transmitted to the same receiver over the same radio link.
To reduce large variations in the instantaneous power of the transmitted power at the WTRU, single carrier frequency division multiple access (SC-FDMA) may be adopted as a long term evolution (LTE) UL communication techniques. An exemplary diagram of an SC-FDMA transmitter 200 is shown in FIG. 2. The main difference between SC-FDMA and orthogonal frequency division multiple access (OFDMA) is that, before feeding into an inverse discrete Fourier transform (IDFT) modulation with a larger number of subcarriers, the signals are discrete Fourier transform (DFT) spread with a smaller number of points. Therefore, SC-FDMA may also be called DFT-spread orthogonal frequency division multiplexing. This scheme decreases variations in the instantaneous power of the transmitted signal, (single-carrier property), but keeps the orthogonal property of subcarriers to combat frequency selectivity and provide the flexibility of bandwidth assignment in FDMA communications.
Consideration is given to using relaying in multicarrier systems to improve the WTRU's signal-to-interference plus noise ratio (SINR) distribution, and further exploitation of frequency diversity in cooperative relaying schemes.
Consideration is given to implementation of relaying schemes in multicarrier communication systems. Due to frequency selectivity, different subcarriers have different fading coefficients. To maximize throughput, it is possible to use relaying for some subcarriers for a given WTRU. For example, relaying may be used for subcarriers which are in a very bad situation, (e.g., those who suffer from high fading coefficients), and direct transmission may be performed for the subcarriers which are in good enough situations. Thus, the RS may help those subcarriers which experience a bad WTRU-BS link. The BS may make per-subcarrier based decisions for the WTRU. These decisions may be based on the effective channel information computed from the channel information of the WTRU and the RS. The BS may schedule subcarriers to different WTRUs, and the BS may decide which subcarrier should be used for a certain WTRU's direct transmission or cooperative transmission through the RS.
An illustrative example of this procedure is given in FIG. 3. The BS scheduling information may be carried on the control channel sent to the WTRU. In a multicarrier cooperative scheme, carriers may be partitioned into two groups: f1, to be used for direct (one-hop) transmission, and f2, to be used for cooperative (two-hop) transmission. The groups f1 and f2 may be treated disjointly. In this case, all the DL cooperative schemes described above may be used for cooperative communication on f2 group carriers. To maximize throughput, it may be required that the BS have feedback for every channel quality, so that f1 and f2 are partitioned well for every assigned WTRU. Therefore, the overhead may be large.
The relay schemes discussed above may be applied in the subcarriers which assigned to two-hop transmission. The BS scheduling and decision can be per-subcarrier based, per-radio bearer (RB) based, (wherein one RB contains continuous subcarriers and each RB is considered to have a flat fading channel), per-subband based, (wherein one subband consists of consecutive RBs, and each subband is considered to have a flat fading channel), or another basis, as long as the unit is considered as having a flat fading channel.
In particular, a relay system implemented with rateless coding may achieve rates approaching theoretical limits without the requirement of CSI at the transmitter. The relay system may also increase the robustness of the variations of channel statistics. System-level simulation results show that a multicast relaying system with rateless coding may provide up to 20% gains over conventional coded relaying systems in terms of cell throughput. Furthermore, compared with conventionally coded relaying systems, less overhead is required between the WTRU, RS, and BS in a rateless coded system. This is advantageous for practical implementation.
It is desirable to achieve similar advantages in UL communications. Therefore, several options are considered for system level architecture for relayed UL communications assuming TDM operation in the RS, where the reception and transmission of the RS cannot be preformed at the same time slot.